1. Field of the Invention
The present invention relates to a wireless local area network (LAN) system, a fault recovery method, and a recording medium stored therein a computer program for executing the fault recovering process. More particularly, the present invention relates to a technology for preventing the throughput of the whole LAN system from being decreased even when a fault occurs in an access point.
2. Description of the Prior Art
Conventionally, there has been a wireless local area network (LAN) system connected to a wired LAN and many wireless terminals.
Japanese Patent Laid-Open Publication No. Hei 7-312597 (disclosed in Japan on Nov. 28, 1995) discloses a wireless LAN system that prevents packets from being lost. This technology will be described below as a first conventional example.
FIG. 10 is a block diagram showing the wireless LAN system according to the first conventional example. A wireless LAN system 110, shown in FIG. 10, consists of a client terminal 118 connected to a wired LAN 112, a LAN cable 120 forming the wired LAN 112, access points 122 and 124 performing as bridges to the wired LAN 112, and wireless terminals 130 and 134 which, in conjunction with the access points 122 and 124, form wireless LANs 114 and 116, respectively.
In the system of this conventional example, when the source wireless terminal 130 transmits a packet to the destination wireless terminal 134 but the wireless terminal 134 does not respond to it, the access point 122 or 124 transmits the packet to the wireless LAN 114 or 116 or transmits the packet to the wired LAN 112. In this way, the system prevents a packet loss caused by a conflict in accessing the medium or an interference with other communication networks using the same frequency band.
Japanese Patent Laid-Open Publication No. Hei 8-242232 (disclosed in Japan on Sep. 17, 1996) discloses a wireless terminal LAN having a repeater. This technology will be described as a second conventional example.
FIG. 11 is a block diagram of the wireless LAN system of the second conventional example. The wireless terminal LAN, shown in FIG. 11, consists of wireless terminals 202-210 and a wireless repeater 201 containing a transceiver 220.
The system according to this conventional example has the special wireless repeater 201 and, through this wireless repeater 201, re-transmits packets to reduce conflicts in radio waves. Thus, the system can prevent the throughput of the whole LAN from being decreased.
Japanese Patent Laid-Open Publication No. Hei 9-215044 (disclosed in Japan on Aug. 15, 1997) discloses a priority switching technology for a cellular wireless LAN. This technology will be described as a third conventional example.
FIG. 12 is a block diagram of the wireless LAN system according to the third conventional example. The wireless LAN system, shown in FIG. 12, consists of a plurality of portable units 302 that are wireless terminals, a house computer 304 connected to a wired LAN, and a plurality of access points 305 that perform as bridges to the wired LAN.
In the system according to this conventional example, the portable unit 302 searches and identifies the access point 305 best suited for communication, based on the intensity of radio waves from, and the loading factor of, the plurality of the access points 305.
In addition, to make the advantages of the present invention clearer, a virtual technology with a configuration equivalent to that of the system according to the present invention will be described as a fourth conventional example.
FIG. 13 is a block diagram of a wireless LAN system according to the fourth conventional example. FIG. 14 is a flowchart showing a fault recovery processing of the wireless LAN system according to the fourth conventional example.
The wireless LAN system, shown in FIG. 13, consists of access points Pa and Pb that perform as bridges to a wired LAN system Lw, client terminals Ca-Cd that are wireless terminals communicating with the access points Pa and Pb, and a LAN cable 405 consisting the wired LAN system Lw.
As shown in FIG. 13, the access points Pa and Pb transmit beacon information Iba and Ibb, respectively, at regular intervals (step S101 in FIG. 14).
When the client terminal Ca receives the beacon information Iba from the access point Pa at power-on time or roaming time, the client terminal Ca transmits a management frame back to the access point Pa to start negotiation with the access point Pa and then starts communication with the access point Pa.
The client terminal Cb also starts negotiation with the access point Pb and then starts communication with the access point Pb according to the same procedure (step S102 in FIG. 14).
At this point, when the access point Pa fails for some reason or other (step S103 in FIG. 14), the client terminal Ca cannot continue communication with the access point Pa. So, the client terminal Ca starts searching for another access point Pb (step S104 in FIG. 14).
If another access point Pb is near the client terminal Ca (step S105 in FIG. 14), there is no problem because the client terminal Ca can immediately establish a link with the access point Pb to continue communication (step S106 in FIG. 14).
However, if another access point Pb is not near the client terminal Ca (step S105 in FIG. 14), the client terminal Ca continues search processing until it successfully searches for the access point Pb (step S104 in FIG. 14).
This search processing is executed by the client terminal Ca for transmitting a management frame (probe) that is communication control management information. To establish the link to the access point Pb as soon as possible and to continue communication reliably, the client terminal Ca continues transmitting the management frame, during this search processing, at an interval shorter than that for a normal communication frame.
However, the above processing has the problems described below.
First, in the above search processing, the client terminal Ca frequently transmits the management frame at an interval shorter than that for the normal communication frame. This increases the radio wave interference around the client terminal Ca and therefore decreases the throughput of the whole LAN system L.
Second, in the above search processing, the client terminal Ca frequently transmits the management frame at an interval shorter than that for the normal communication frame. This processing has some problems with the client terminal Ca being battery-powered. For example, this processing consumes more battery power for communication and therefore shortens battery life. And, the increase in power consumption causes a quickly decrease in the power voltage, sometimes suddenly disconnecting the power of the portable information terminal and thus destroying data due to a communication interruption.
Third, in a wireless LAN system Ll, the duplicated system configuration is built usually for the access points Pa and Pb as a fail-safe against the shutdown of the whole LAN system L. This shutdown may be caused by such conditions as a fault in access point Pa or Pb or the disconnection of a LAN cable 405. However, in a wireless LAN system used generally in Japan, whose frequency bandwidth is only one third of a wireless LAN system generally used in the United States, duplicating the access points Pa and Pb allows the access points Pa and Pb to transmit radio waves frequently during the above search processing. This increases the radio wave interference and therefore decreases the throughput of the whole LAN system.